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Are Your Glue Joints Repairable?

Have you tested your favorite wood glue to see if it can be reglued successfully should either your work be damaged, or a cross-grain glue joint fail with age and seasonal movement?

In 4 decades as a woodworker, I’ve done a good bit of conservation, repair and restoration work, including pieces in a few federal museums both here and overseas. As I pass what I know down to my boys, included will be what I know about glues. I know that some glue types can’t be glued over, often requiring new wood to be let in during repairs, and the joint recut. I discovered that the hard way some decades ago restoring furniture, and simply switched to other glues for all my work. Since then, those glues I rejected may have been reformulated; plus there are a number of new glues worth checking out, so to make sure I’m not providing bad or outdated advice, it’s time to check out the current crop of wood glues for repairability.

I make no pretenses toward science, here…this is all anecdotal based on experience, not chemistry…all I want to show is whether marine epoxy will adhere to the glue lines or residue of the various wood glues during repairs. You can look up strength and other test data in your USDA Wood Handbook; I care about repairability because I’ve never seen any test or even anecdotal data on anything but hide glue in that regard, and it’s important if your work is to survive beyond typical damage and wear and tear over time. I chose epoxy as the regluing agent because it’s the usual choice in professional structural repair work and it adheres to a greater number of diverse substances than any other wood glue I know of. In fact, it usually rebonds a failed but fully cured glue joint much better than the original glue would, and as it also bonds to itself very well, epoxy is a good, repairable choice for many applications.

On identical tiles of freshly planed, vertical grain, second-growth Doug Fir, I saturated the faying surfaces with glue and let them cure to full strength by the manufacturer’s instructions for time and temperature….

…then I keyed each faying surface with 100-grit abrasive paper, reglued them with marine epoxy, and “clamped” the assemblies to the degree favored by epoxy. For glues that left a rough surface like polyurethane, the epoxy was applied twice…an unthickened coat followed by a second coat thickened with West 404 High-Adhesive Thickener, per the manufacturer’s instructions. I let the epoxy cure for 6 days to reach full strength.

I purposely chose small blocks of wood with easily broken short grain because strength here isn’t the issue, adherence is, and I can check adherence using a sharp chisel without trying to break long glue joints in a press. Of greater concern was that the glues to be tested were applied without any clamping pressure, but as it turned out, several glues that require high clamping pressure fared very well, so I believe the results are reasonably valid.

The results offered no surprises.

The epoxy thoroughly adhered to the strongest of the off-the-shelf glues, the 2-part resorcinol, breaking completely at the wood rather than the glue line. Attempts to slip the chisel between the glue lines revealed a thorough and unified bond between all three layers of glue.

Epoxy on epoxy showed similar results……and so did liquid polyurethane (Elmer’s Ultimate)…

…and powdered urea formaldehyde plastic resin glue.

Titebond, a Poly Vinyl Acetate glue, however, broke some wood but failed the chisel test…. the chisel easily separated the two layers of Titebond, indicating poor adherence of the epoxy in between.

Titebond II broke even less wood, with poor adherence…

…and Titebond III, while a much stronger glue, still did not adhere to the epoxy.

The implications of all this can be minor if we are talking about a first-effort coffee table….but they can be serious if we are talking about a strip-planked boat hull made of 1 X 1 strips glued together using an unrepairable glue. Picture the requirement to feather in a large patch to repair hull damage, and you can see that patch will be pinstriped with unsound repair at every glue line, leading to early failure of the repair.

You can draw your own conclusions. Mine are that the work most easily restored is often the work that survives the longest, that you may not care about longevity, but that may break you granddaughter’s heart some day, and I’d check out my glue choices thoroughly before committing them to any 20-hour high-end project, let alone a 700-hour project.

““Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

When in doubt I always reach for West System. One thing about Epoxy (or anything else) to cured Epoxy, it can't hurt to wipe it down with lacquer thinner before, re-coating or bonding. When Epoxy cures it releases amines, a wax like substance that ends up covering the surface, which could cause finishing and/or bonding problems in the next stage. Old boatmaker 'splained that to me, who also taught me my love for epoxy. make sure you work in a well vetilated area, though, when working with it. Smells rather un-offensive, almost pleasant, but can be very nasty to the respiratory system.

I use West because I'm thoroughly familiar with it, and as a traditional builder, I only use a gallon or so a year, so expense isn't an issue.

But System III, MAS and others make epoxies that don't suffer amine blush, which also washes off with plain soap and water.

““Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

Thanks Bob for passing along your experience. This has me re-evaluating Titebond III as my primary glue. I also use polyurethane for joints that require maximum strength. Despite other reviews, it seems to have better than average gap filling ability (foam). The down side of this glue is that the foam squeeze out is difficult to control.I suspect that others are not letting polyurethane cure long enough before performing strength tests.

I would conclude that your choice is epoxy, especially for boat building.

Liquid poly isn't near as strong as epoxy and that foam may fill a gap but it provides zero strength, unlike thickened epoxy.

Personally, I use U/F resin glue in all indoor work, poly in most outdoor work and resorcinol and epoxy in boats.

These are the first bottles of Titebond I've bought in 3 decades. Nothing I know of in this world adheres to cured Titebond. And I've tried most of them.

The usual scenario was a fine, old, round-tenon chair assembled with hide glue. Hide glue is a fully-renewable glue where all you have to do to reglue the joint is soak it in fresh, hot hide glue....30 to 60 minutes to reglue the whole durn chair like new. That's why the builder used a glue that dissolves itself, as the nature of round-tenon joints is that they usually break their glue bonds every 20 years or so from seasonal movement, regardless of what glue was used.

But the owner reglued it with whatever PVA glue he had on hand...and that repair lasted a couple weeks. After a few cycles of that, he brings it to me...and it costs him big bucks, because the only way I can repair it is to steam the entire chair apart, then plug and recut the joints he contaminated with PVA.

As you can imagine, often the chair became kindling upon receipt of the estimate.

““Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

Hmmm, do you build now with aliphatic glues or build for the future where it might break and be easily repairable? I'm not really in the forecasting business of where a project of mine might break...I can only worry so much. I think I would prefer to focus on building high-quality with sound wood and joint choices.

Epoxies are great for what they do but they aren't the easiest glues to use and they aren't cheap...neither is poly for that matter although it appears to pass your testing.

I think an article is called for here and to get the chemists involved to see what the explanation for our favorite yellow glues failing the Smalser repairable test!

Hmmm, do you build now with aliphatic glues or build for the future where it might break and be easily repairable?

The pics don't lie, and I'm sure no Franklin International chemist will talk about it.

From my experience doing restoration work, if you are using Titebond, you are doing relatively short-lived work. Things that can't be economically reparied usually become kindling.

Every crossgrain and round tenon gluejoint in your work will eventually break its glue bond. Long grain layups last forever unless something smashes them and usually aren't as big a concern.

There are probably 20 furniture items in this house done by my hand, and an equal number of like doodads in the shop...not to mention work sold. They were all done with plastic resin, which is much cheaper than Titebond, and there hasn't been a glueline failure in the 30 years I been doing it.

But when those crossgrain joints do fail, any Granddaughter with a tube of household epoxy and an inner tube clamp can fix them for the next hundred years.

Last edited by Bob Smalser; 06-28-2005 at 6:13 PM.

““Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

If you can mix a cup of cocoa, you can mix plastic resin or resorcinol. If you can pump mustard on a hot dog, you can mix epoxy.

The only down sides of plastic resin is that it likes warmer temps than PVA's....and that it has a shelf life of 1 year. As my PVA's freeze long before than, it ain't an issue with me.

““Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

When I started building things, I never considered the implications of ignoring time. Over the years, I have developed an appreciation for time. I take seriously the duration of the life of anything that I build. Good construction, assembly, and finishing practices have moved into so much of what I build. I have found that unlike some things from the store, crafted items tend to hang around forever.

I built a set of open bookcases for a daughter over a decade ago. She showed me a picture and asked for something like this for her room. I put them together one afternoon. I figured that she would dump them when she went off to college and I could pitch them out. In reality, these cases have been to and from college, to one house after another in moving all over the country, and reside in her house now. Except for some scratches, the past 16 or so years have not done them in. Last year, my younger daughter wanted a set "just like hers". You can be sure that more time and care went into these.

I have built much furniture and fixtures for churches, so the original time frame has been pretty much forever. When I build the interior of a sanctuary, I consider that it may well be in continuous use for over 100 years. I still have all the as-built drawings for all these churches. I try to build these so that there should never need a repair short of a disaster such as a fire or tornado.

This information about glues is quite informative. I wish that I had known it all along.

Well, the good news for PL users is that something certainly sticks to PL Premium poly construction adhesive.

Problem is, I neglected to write down whether it wuz epoxy or more PL and I'll have to run the test again.

““Perhaps then, you will say, ‘But where can one have a boat like that built today?’ And I will tell you that there are still some honest men who can sharpen a saw, plane, or adze...men (who) live and work in out of the way places, but that is lucky, for they can acquire materials for one third of city prices. Best, some of these gentlemen’s boatshops are in places where nothing but the occasional honk of a wild goose will distract them from their work.” -- L Francis Herreshoff

Bob, thank you for the lesson on glues. I, for one, am going to pay a lot more attention to which glue I'm using where and why I'm using it. You lost me on the last post, what's PL? I appreciate your willingness to share your knowledge, Thank you.

thanks bob for the glue info

know i have a question for ya, how does the plastic resin glue do in reguards to the finishing aspect. does the joint line show up? clean up in reguards to the surface prep for stain or clear finish. are there some tricks that you have learned in the respect that you could share? thanks in advance, i had used that plastic resin in school but hadnt since maybe the teacher was on to somethng? i figured it was just because it worked with the heater they had to dry the joints.